Method and Device for Detecting a Neural Response in Neural Measurements

ABSTRACT

A method is provided for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement. A first neural measurement is obtained from a first sense electrode. A second neural measurement is contemporaneously obtained from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response. A neural response decay is determined, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode. A ratio of the neural response decay to an amplitude normalising term is calculated. From the ratio it is determined whether a locally evoked neural response is present in the neural measurement.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Australian Provisional Patent Application No. 2014904595 filed 17 Nov. 2014, which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to detection of a neural response, such as a neural response caused by a stimulus. In particular the present invention relates to detection of a compound action potential by using one or more electrodes implanted proximal to the neural pathway to obtain a neural measurement.

BACKGROUND OF THE INVENTION

Electrical neuromodulation is used or envisaged for use to treat a variety of disorders including chronic pain, Parkinson's disease, and migraine, and to restore function including but not limited to hearing and motor function. A neuromodulation system applies an electrical pulse to neural tissue in order to generate a therapeutic effect. Such a system typically comprises an implanted electrical pulse generator, and a power source such as a battery that may be rechargeable by transcutaneous inductive transfer. An electrode array is connected to the pulse generator, and is positioned close to the neural pathway(s) of interest. A suitable electrical pulse applied to the neural pathway by an electrode causes the depolarisation of neurons, which generates propagating action potentials whether antidromic, orthodromic, or both, to achieve the therapeutic effect.

When used to relieve chronic pain for example, the electrical pulse is applied to the dorsal column (DC) of the spinal cord and the electrode array is positioned in the dorsal epidural space. The dorsal column fibres being stimulated in this way inhibit the transmission of pain signals through that segment in the spinal cord to the brain.

In general, the electrical stimulus generated in a neuromodulation system triggers a neural action potential which then has either an inhibitory or excitatory effect. Inhibitory effects can be used to modulate an undesired process such as the transmission of pain, or excitatory effects can be used to cause a desired effect such as the contraction of a muscle or stimulation of the auditory nerve.

The action potentials generated among a large number of fibres sum to form a compound action potential (CAP). The CAP is the sum of responses from a large number of single fibre action potentials. When a CAP is electrically recorded, the measurement comprises the result of a large number of different fibres depolarising. The propagation velocity is determined largely by the fibre diameter and for large myelinated fibres as found in the dorsal root entry zone (DREZ) and nearby dorsal column the velocity can be over 60 ms⁻¹. The CAP generated from the firing of a group of similar fibres is measured as a positive peak P1 in the recorded potential, then a negative peak N1, followed by a second positive peak P2. This is caused by the region of activation passing the recording electrode as the action potentials propagate along the individual fibres, producing the typical three-peaked response profile. Depending on stimulus polarity and the sense electrode configuration, the measured profile of some CAPs may be of reversed polarity, with two negative peaks and one positive peak.

To better understand the effects of neuromodulation and/or other neural stimuli, and for example to provide a stimulator controlled by neural response feedback, it is desirable to accurately detect a CAP resulting from the stimulus. Evoked responses are less difficult to detect when they appear later in time than the artifact, or when the signal-to-noise ratio is sufficiently high. The artifact is often restricted to a time of 1-2 ms after the stimulus and so, provided the neural response is detected after this time window, a response measurement can be more easily obtained. This is the case in surgical monitoring where there are large distances (e.g. more than 12 cm for nerves conducting at 60 ms⁻¹) between the stimulating and recording electrodes so that the propagation time from the stimulus site to the recording electrodes exceeds 2 ms.

To characterize the responses from the dorsal columns, high stimulation currents and close proximity between electrodes are required, and therefore in such situations the measurement process must overcome artifact directly. However, this can be a difficult task as an observed CAP signal component in the neural measurement will typically have a maximum amplitude in the range of microvolts. In contrast a stimulus applied to evoke the CAP is typically several volts and results in electrode artifact, which manifests in the neural measurement as a decaying output of several millivolts partly or wholly contemporaneously with the CAP signal, presenting a significant obstacle to isolating or even detecting the much smaller CAP signal of interest.

For example, to resolve a 10 uV CAP with 1 uV resolution in the presence of an input 5 V stimulus requires an amplifier with a dynamic range of 134 dB, which is impractical in implant systems. As the neural response can be contemporaneous with the stimulus and/or the stimulus artefact, CAP measurements present a difficult challenge of measurement amplifier design. In practice, many non-ideal aspects of a circuit lead to artefact, and as these mostly have a decaying exponential appearance that can be of positive or negative polarity, their identification and elimination can be laborious.

The difficulty of this problem is further exacerbated when attempting to implement CAP detection in an implanted device. Typical implants have a power budget which permits a limited number, for example in the hundreds or low thousands, of processor instructions per stimulus, in order to maintain a desired battery lifetime. Accordingly, if a CAP detector for an implanted device is to be used regularly (e.g. once a second), then the detector should preferably consume only a small fraction of the power budget and thus desirably should require only in the tens of processor instructions in order to complete its task.

Approaches proposed for obtaining a neural measurement include that described in International Patent Publication No. WO 2012/155183. Approaches to identifying whether a neural response is present in a neural measurement include the propagram method described in International Patent Publication No. WO 2012/155190.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

In this specification, a statement that an element may be “at least one of” a list of options is to be understood that the element may be any one of the listed options, or may be any combination of two or more of the listed options.

SUMMARY OF THE INVENTION

According to a first aspect the present invention provides a method for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement, the method comprising:

obtaining a first neural measurement from a first sense electrode;

obtaining a contemporaneous second neural measurement from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response;

determining a neural response decay, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode;

calculating a ratio of the neural response decay to an amplitude normalising term; and

determining from the ratio whether a locally evoked neural response is present in the neural measurement.

According to a second aspect the present invention provides an implantable device for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement, the device comprising:

measurement circuitry for obtaining a first neural measurement from a first sense electrode, and for contemporaneously obtaining a second neural response measurement from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response; and

a processor configured to determine a neural response decay, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode; the processor further configured to calculate a ratio of the neural response decay to an amplitude normalising term; and the processor further configured to determine from the ratio whether a locally evoked neural response is present in the neural measurement.

According to a third aspect the present invention provides a non-transitory computer readable medium for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement, comprising instructions which, when executed by one or more processors, causes performance of the following:

obtaining a first neural measurement from a first sense electrode;

obtaining a contemporaneous second neural measurement from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response;

determining a neural response decay, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode;

calculating a ratio of the neural response decay to an amplitude normalising term; and

determining from the ratio whether a locally evoked neural response is present in the neural measurement.

The present invention recognises that a locally evoked neural response, being a neural response evoked at a site close to the sense electrodes, such as within 100 mm or less, will undergo a decay in the CAP signal amplitude as it propagates away from the stimulus site, due at least in part to a spreading in time of the responses of individual fibres having differing conduction velocity, and a change in depth of individual fibres within the spinal cord as the CAP propagates. This is in contrast to a distally evoked response, which will present a substantially constant neural signal strength to both sense electrodes. Accordingly the present invention provides a way to determine whether a sensed neural response has been locally evoked or distally evoked. Such a capability may thus be used for example to characterise the performance of a local electrical stimulus regime, without corruption from noise such as coexisting neural responses occurring on the neural pathway due to the subject's independent motor activity and/or sensory stimulation. Such distally evoked responses give rise to considerable neural activity and, without a means for distinguishing between locally evoked responses and distally evoked responses, distally evoked response noise can lead to a conclusion that locally applied electrical stimuli are performing appropriately when in fact they are not.

The neural response decay may in some embodiments be determined by determining a first amplitude of the first neural measurement, determining a second amplitude of the second neural measurement, and calculating a difference between the first amplitude and the second amplitude. In such embodiments, the CAP amplitude may be determined in any suitable manner, for example in accordance with the teachings of Australian Provisional Patent Application No. 2013904519, the content of which is incorporated herein by reference. Additionally or alternatively, the neural response decay may in some embodiments be determined by determining a first width of the first neural measurement, determining a second width of the second neural measurement, and calculating a difference between the first width and second width. Still further embodiments may determine the ratio as being a ratio of the amplitude (or energy, power of other strength measure) of the first neural measurement to the amplitude (or energy, power of other strength measure) of the second neural measurement.

The amplitude normalising term may in some embodiments of the invention comprise a sum of the first amplitude and second amplitude, a sum of scalar or other variants of the first amplitude and second amplitude, a scalar or other variant of the first amplitude alone, or a scalar or other variant of the second amplitude alone. The present invention recognises that such normalisation of the difference value is an important element of detecting neural responses because of the propensity of spinal cord electrode arrays to move relative to the spinal cord and alter the electrode-to-fibre distance, and because of the impact of the electrode-to-fibre distance upon both (i) the amplitude of the response evoked by a given stimulus, and (ii) the amplitude of a neural measurement obtained from a given neural response.

The first and second amplitudes are preferably determined at a moment of the respective measurement corresponding to an expected occurrence of a neural response to be detected, as determined by reference to an electrical stimulus timing and a distance from the stimulus site to the respective sense electrode.

In some embodiments the measurement is obtained in accordance with the teachings of International Patent Publication No. WO 2012/155183, by the present applicant.

In some embodiments the detector output is used in a closed loop feedback circuit to control neuromodulation, for example in conjunction with the techniques of International Patent Publication No. WO 2012/155188, by the present applicant, the content of which is incorporated herein by reference. Such embodiments may thus effect feedback control of electrical stimuli with improved resistance to corruption by the patient's independent motor activity and/or peripheral stimuli.

In some embodiments the method may be repeated in order to obtain a plurality of ratios resulting from repeated application of a given stimulus. The plurality of ratios may then give a probabilistic indication of the neural response decay to improve the determination of whether a locally evoked response is present.

In some embodiments, the method may be performed repeatedly, regularly or substantially continuously, in order to monitor changes in the ratio which occur over time, for example in response to postural changes of the subject, movement of the subject, peripheral stimuli experienced by the subject, electrode lead movement, injury or disease affecting the neural pathway, or a change in efficacy of a therapy such as medication.

In some embodiments the method of the present invention may further comprise the step of detecting whether any neural activity is present. Such embodiments recognise that insufficiently suppressed stimulus artefact also decays with distance from the stimulus site and the ratio may thus give a false positive indication that a locally evoked response is present, when in fact only artefact is present. By providing a separate step of detecting whether neural activity is present, such embodiments may provide improve performance in embodiments in which stimulus artefact is inadequately or not suppressed. Such embodiments may comprise a signal quality indicator configured to assess the neural measurement(s) in order to determine whether a signal appears to be a CAP, and if not to exclude the measurement from further processing.

In some embodiments, a contemporaneous third or additional neural measurement may be obtained from a third or additional sense electrode(s) spaced apart from the first and second electrodes along a neural pathway of the neural response. Such embodiments may be used for three or more point fitting of a decay coefficient of the observed response, for use as a determinant of whether an observed response has been locally evoked.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates an implantable device suitable for implementing the present invention;

FIG. 2 is a schematic of a feedback controller to effect stimulus control in response to neural recruitment in accordance with one embodiment of the present invention;

FIG. 3 illustrates the reduction in observed amplitude of an evoked neural response with increasing distance from the stimulus site;

FIG. 4a illustrates the amplitude of observed neural responses of a first patient, FIG. 4b is a time series of histograms of neural response decay values (R values) derived from FIG. 4a , FIG. 4c illustrates a single histogram of signal, and noise, respectively, from the same patient and FIG. 4d illustrates the time series of the R histograms obtained in the absence of any locally evoked signals;

FIG. 5a illustrates selection of the upper and lower boundaries of an inclusion criterion, and FIG. 5b illustrates for each width setting the percentage of consecutive non-usable points;

FIG. 6a is a time series of histograms of R values for evoked response signals for a second patient, FIG. 6b is a time series of histograms for the same patient in the presence of noise only, and FIG. 6c illustrates the performance of differing inclusion criteria;

FIG. 7a is a time series of histograms of R values for evoked response signals of a third patient, FIG. 7b illustrates the histogram of R values obtained in the presence of noise only for the same patient, and FIG. 7c illustrates the performance of differing inclusion criteria;

FIG. 8 illustrates rejection of neural measurements using the inclusion criteria; and

FIG. 9a is a plot of neural amplitude response data from a fourth patient, FIG. 9b is a plot of the R data obtained from FIG. 9a , and FIG. 9c is a plot of the R data obtained from noise during the same period of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an implantable device 100 suitable for implementing the present invention. Device 100 comprises an implanted control unit 110, which controls application of neural stimuli, and controls a measurement process for obtaining a measurement of a neural response evoked by the stimuli from each of a plurality of electrodes. The control unit 110 includes a processor and a storage memory (or other storage device(s), not shown) for carrying out the method of the present embodiment of the invention. Device 100 further comprises an electrode array 120 consisting of a linear array of electrodes 122, each of which may be selectively used as either the stimulus electrode or sense electrode, or both.

FIG. 2 is a schematic of a feedback controller implemented by the control unit 110, based on recruitment. An important component of such feedback control is a recruitment estimator 230, which performs the difficult task of detecting whether an evoked neural response is present in a neural measurement as a result of the electrical stimulus.

In this embodiment, electrical stimuli are delivered to the spinal cord 202 by one or more stimulus electrodes denoted E1 in FIG. 2. A desired degree of recruitment, A R_(desired), is input by the user or by a setting made by a clinician when fitting the device or by any other suitable means for defining desired recruitment. R_(desired) is processed by a controller and selector and passed to a stimulus generator which generates a stimulus to be delivered to the neural tissue by E1. As will be appreciated while only a single stimulus electrode E1 is shown in FIG. 2, a bipolar, monopolar or tripolar stimulus may be applied in conjunction with other stimulus electrodes, not shown. At the stimulus site adjacent to E1 within the spinal cord 202, a neural response is evoked by the stimulus.

The neural response evoked by the stimulus at E1 is a compound response comprising the individual responses evoked in a number of fibres, and takes a form shown at 210. The evoked response 210 propagates along the recruited fibres within the spinal cord 202 away from the stimulus site adjacent to E1, and in so doing the form or morphology of the compound response alters or decays. Without intending to be limited by theory, the decay in the neural response as it travels is at least in part due to a spreading of the compound response resulting from each recruited fibre having a conduction velocity which differs from the conduction velocity of other recruited fibres, and the variation in depth of the recruited fibres within the cord 202 at different positions along the cord. At a time t2 the compound response passes sense electrode E2 and is recorded as having an amplitude and duration indicated at 212, which differs from the form of the response at 210 in that response 212 is of reduced amplitude and greater width or duration. At a later time t3, after undergoing further spreading and decay, the compound response passes sense electrode E3 and is recorded as having an amplitude and duration indicated at 214. Observed response 214 is of lesser amplitude but greater duration then observed response 212. Similarly, at a later time t4, after undergoing further spreading and decay, the compound response passes electrode E4 and is recorded as having a further decreased amplitude and increased duration as indicated at 216.

It is to be appreciated that the form of each observed response, as shown at 210, 212, 214 and 216, is illustrative. The decay and spreading observed in any neural response will depend at least upon the characteristics of the fibre population actually recruited by the stimulus, the neurophysiology of the subject, and the distance of the electrodes from the fibres.

In accordance with the present invention, electrodes E2 and E3 are used to obtain a first measurement 212 and a second measurement 214 of the neural response evoked by the stimulus, via measurement circuitry 222, 224 respectively. The evoked CAP measurements in this embodiment are made by use of the neural response measurement techniques set out in International Patent Publication No. WO2012/155183, with two data channels recording simultaneous data from the two electrodes E2 and E3.

Applying a filter with optimized frequency and delay on each channel, in accordance with the teachings of Australian Provisional Patent Application No. 2013904519, the amplitude of the signals on each channel are determined and denoted as a pair of measurements CH1, CH2. Such amplitude measurement pairs are obtained repeatedly over time for each applied stimuli. For each pair of measurements, a neural response decay value R is determined as follows:

$R = \frac{{{CH}\; 1} - {{CH}\; 2}}{{{CH}\; 1} + {{CH}\; 2}}$

FIG. 3 illustrates the reduction in observed amplitude of an evoked neural response with increasing distance from the stimulus site. As can be seen the absolute difference between adjacent electrode recordings is larger when closer to the stimulus site, hence the selection of E2 and E3 as sense electrodes in this embodiment, however other electrode pairs such as E3 and E4 or E2 and E4 may be used in alternative embodiments of the present invention. Once sufficiently distant from the stimulus site which depending on the patient may be around 100 mm from the stimulus, and as indicated at 302, the amplitude remains at a constant non-zero level due to the nature of neural propagation. Thus, if a neural response is evoked at such a distance or further away from the sense electrodes, the observed amplitudes will be equal at each electrode and the value of R will be zero (assuming no noise), or randomly spread in the presence of noise. On the other hand, when a neural response is evoked locally to and a constant distance from the sense electrodes E2 and E3, as is the case during effective stimulation from E1, R will take a constant non-zero value in the absence of noise, or will be distributed about that non-zero value in the presence of noise in a manner distinguishable from the values of R in the region 302.

FIG. 4a illustrates the amplitude of observed neural responses from a patient over a period of about 90 seconds. As can be seen, the feedback variable, being the response amplitude sensed by electrode E2, varies considerably over time for example with changes in user posture, motor activity (movement), and peripheral sensory input. Similar data (not shown) is obtained simultaneously from a second electrode E3 spaced further away from the stimulus site E1 than the first electrode E2. In accordance with the present embodiment, during this time period E1 repeatedly delivered a stimulus, and R was determined for every iteration. A histogram was then built of the calculated values of R within a 3 second interval. This is repeated in the next 3 second interval to build a further histogram, with many such histograms being built over an extended period to generate the data shown in FIG. 4b . In this case of a locally evoked response, a band 402 can be seen in FIG. 4b , where most of the ratios (R values) lie. In the present embodiments the measurement circuitry 222, 224 have differing offsets so that the curve of FIG. 3 is not followed, and in particular in the present embodiments this results in the Ratio taking a negative value despite what might be expected from FIG. 3 if the measurements had no offset. However the present invention is advantageous in this regard as it is the tightness or existence of a band which can be determined, irrespective of the position of the band, allowing such measurement offsets to be retained for measurement optimisation. In cases where the measurements have no offset, it can be deduced from FIG. 3 that Ri=0 indicates CH_(1i)=CH_(2i), while Ri=1 indicates that CH_(1i)>>CH_(2i). In this embodiment the dominant value of Ri is around −0.55, as seen in FIG. 4b . This band is then deemed to define the range in which an amplitude measurement is considered valid for this patient, so that an observed response which produces an R in this range, or multiple observed responses which produce an averaged R in this range, is deemed to have been a locally evoked neural response and not a distally evoked neural response.

FIG. 4c also illustrates a histogram 406 of R which was obtained by making multiple measurements each at a time when no response was locally evoked. Histogram 406 illustrates that when measuring random noise or distally evoked neural signals, while they may occasionally produce an R within the band 402 they will not be consistently within it.

FIG. 4d illustrates the time series of the R histograms 406 obtained in the absence of any locally evoked signals. As can be seen in FIG. 4d , even though it is not known whether the sense electrodes are sensing distally evoked neural responses or merely noise, either situation is clearly differentiated from the tightly banded R data produced by locally evoked responses as shown in FIG. 4b . This shows that noise (which could comprise ascending or descending neural signals of remote origin, i.e. originating at the periphery or at the brain or otherwise originating more than a few centimeters away from the measurement electrodes) is not correlated and any neural activity is not synchronised to the stimulus. Thus the present invention provides a means by which to differentiate between a locally evoked CAP (e.g evoked by application of an electrical stimulus from an adjacent electrode) and a distally originating CAP (e.g. a CAP evoked by brain activity, reflex activity or peripheral sensory inputs).

This technique requires the clinician to calibrate the boundary of range 402 or the like for each patient, by performing a feedback experiment to determine the required band for that patient within which the device is accurately measuring a locally evoked CAP and not a distally evoked response.

The selection of the upper and lower boundaries of range 402 was further investigated. FIG. 5a illustrates the data of FIG. 4b with a number of candidate ranges illustrated, as indicated at 502. The range 502 was centred about the mean value of R, and the width of the range 502 was varied in increments of 0.05. FIG. 5b illustrates, for each width setting of range 502, the percentage of consecutive non-usable points, being those points for which it is determined that the amplitude calculated is not due to an evoked CAP, which arise for that inclusion criteria.

It is further noted that, given the variability from one patient to the next in the implantation site, electrode to fibre distance, and other parameters, the absolute and relative response amplitudes observed can vary considerably between patients. FIG. 4a shows a typical response amplitude of around 100 μV, whereas FIG. 6a illustrates data from a patient for whom the typical response amplitude was around 1,300 μV. FIG. 6a shows that a patient for whom large response measurements (>100 μV) are obtained, gives a sharper histogram and a narrower band 602, as compared to the band 402 in FIG. 4b . However, the patient of FIG. 6a exhibited a slower decay of neural response between the two electrodes (E3 and E4 in this instance), so that the R values are located much closer to zero. As can be seen with reference to FIG. 6b , the distributions of R values obtained in the presence of noise only can be easily distinguished from the tightly clustered data of FIG. 6a . FIG. 6c , illustrating the performance of differing inclusion criteria for range 602, also illustrates the improved sharpness of the histogram for the patient of FIG. 6 as compared to the patient of FIG. 4. This likely indicates not only that the original signals were smaller, but also that a higher proportion of the amplitudes measured for the patient of FIG. 6 contained neural signals.

FIG. 7a illustrates R data for a patient for whom the amplitude of the measured responses to electrical stimuli were much smaller, only around 30 μV. As can be seen, the smaller absolute amplitudes result in a broader histogram 702, which is nevertheless clearly distinguishable from the noise-only histograms shown in FIG. 7b . FIG. 7c , illustrating the performance of differing inclusion criteria for range 702, also illustrates the wider spread of the histogram for the patient of FIG. 7 as compared to the respective patients of FIGS. 4 and 6.

FIG. 7c indicates that an inclusion criteria of 0.2, spanning a range of R values of −0.1 to −0.3, will return a false negative for about 15% of response measurements. FIG. 8 illustrates rejection of neural measurements using the inclusion criteria, by rejecting those whose R values fall outside of the set band. In particular FIG. 8 is an extract of the neural response amplitudes returned from one of the electrodes, with the dotted trace indicating the original amplitude values, and the solid trace indicating the output once those measurements deemed to be false measurements by the inclusion criteria are excluded. As expected, about 15% of measurements are rejected with the inclusion criteria set to such values. This illustrates that even for a patient from whom very small (˜30 μV) response amplitudes are obtained, the method of the present embodiment is capable of identifying locally evoked neural responses with a useful degree of selectivity.

The effect of neural response amplitude upon the band of R values was investigated. FIG. 9a is a plot of neural amplitude response data obtained over a 600 second period, with the target value (desired neural response amplitude) being stepped upwards progressively from about 100 seconds to 600 seconds, and the observed neural response amplitude (the feedback variable) rising similarly, albeit with typical variations from noise, posture, etc.

FIG. 9b is a plot of the R data obtained from the E3 and E4 electrodes during the period portrayed in FIG. 9a . As can be seen, the effect of increasing the stimulus current and thereby increasing the neural response amplitude is to increase the amplitude of the histogram, but this makes no difference to the centre position (mean R value) of the histogram. FIG. 9c illustrates noise data obtained during the same period but at times between stimuli when a locally evoked response is not expected to arise, illustrating that distally evoked responses continue to be distinguishable from a locally evoked response even with varying stimulus amplitude.

The present embodiments assume that the energy/amplitude ratio of the CAP as it propagates across electrodes is consistently within a certain range for each patient. However, alternative embodiments may take measures which allow for variations in the energy/amplitude ratio. For example, such alternative embodiments may implement a low resolution sliding time window and histogram calculation to determine if the peak has shifted.

The present embodiment also assumes that what the estimator 230 picks up is in fact a CAP most of the time. In alternative embodiments, where it is possible that a tight histogram of R values might be returned due to the presence of a constant artefact on both channels rather than due to the presence of a decaying neural response, a signal quality indicator may be integrated in order to exclude measurement pairs which are not of the typical three lobed shape of a neural response, for example.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not limiting or restrictive. 

1. A method for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement, the method comprising: obtaining a first neural measurement from a first sense electrode; obtaining a contemporaneous second neural measurement from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response; determining a neural response decay, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode; calculating a ratio of the neural response decay to an amplitude normalising term; and determining from the ratio whether a locally evoked neural response is present in the neural measurement.
 2. The method of claim 1 when used to characterise the performance of a local electrical stimulus regime.
 3. The method of claim 1 wherein the neural response decay is determined by determining a first amplitude of the first neural measurement, determining a second amplitude of the second neural measurement, and calculating a difference between the first amplitude and the second amplitude.
 4. The method of claim 1 wherein the neural response decay is determined by determining a first width of the first neural measurement, determining a second width of the second neural measurement, and calculating a difference between the first width and second width.
 5. The method of claim 1 wherein the ratio is determined as being a ratio of the amplitude or strength of the first neural measurement to the amplitude or strength of the second neural measurement.
 6. The method of claim 3 wherein the amplitude normalising term comprises a sum of scalar variants of the first amplitude and second amplitude.
 7. The method of claim 3 wherein the first and second amplitudes are determined at a moment of the respective measurement corresponding to an expected occurrence of a neural response to be detected, as determined by reference to an electrical stimulus timing and a distance from the stimulus site to the respective sense electrode.
 8. The method of claim 1 wherein the method is repeated in order to obtain a plurality of ratios resulting from repeated application of a given stimulus in order to give a probabilistic indication of the neural response decay to improve the determination of whether a locally evoked response is present.
 9. The method of claim 1 wherein the method is performed repeatedly in order to monitor changes in the ratio which occur over time.
 10. The method of claim 1 further comprising the step of using a signal quality indicator to detect whether any neural activity is present to avoid false positives.
 11. The method of claim 1 further comprising obtaining a contemporaneous third or additional neural measurement from a third or additional sense electrode(s) spaced apart from the first and second electrodes along a neural pathway of the neural response.
 12. An implantable device for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement, the device comprising: measurement circuitry for obtaining a first neural measurement from a first sense electrode, and for contemporaneously obtaining a second neural response measurement from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response; and a processor configured to determine a neural response decay, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode; the processor further configured to calculate a ratio of the neural response decay to an amplitude normalising term; and the processor further configured to determine from the ratio whether a locally evoked neural response is present in the neural measurement.
 13. A non-transitory computer readable medium for processing a neural measurement obtained in the presence of noise, in order to detect whether a locally evoked neural response is present in the neural measurement, comprising instructions which, when executed by one or more processors, causes performance of the following: obtaining a first neural measurement from a first sense electrode; obtaining a contemporaneous second neural measurement from a second sense electrode spaced apart from the first electrode along a neural pathway of the neural response; determining a neural response decay, being a measure of the decay in the neural response from the first sense electrode to the second sense electrode; calculating a ratio of the neural response decay to an amplitude normalising term; and determining from the ratio whether a locally evoked neural response is present in the neural measurement.
 14. The implantable device of claim 12, wherein the processor is configured to determine the neural response decay by determining a first amplitude of the first neural measurement, determining a second amplitude of the second neural measurement, and calculating a difference between the first amplitude and the second amplitude
 15. The implantable device of claim 14, wherein the processor is configured to calculate the amplitude normalising term as a sum of scalar variants of the first amplitude and second amplitude.
 16. The implantable device of claim 12, wherein the processor is configured to determine the neural response decay by determining a first width of the first neural measurement, determining a second width of the second neural measurement, and calculating a difference between the first width and second width.
 17. The implantable device of claim 12, wherein the processor is configured to determine the ratio as being a ratio of the amplitude or strength of the first neural measurement to the amplitude or strength of the second neural measurement.
 18. The implantable device of claim 12, wherein the processor is configured to determine the first and second amplitudes at a moment of the respective measurement corresponding to an expected occurrence of a neural response to be detected, as determined by reference to an electrical stimulus timing and a distance from the stimulus site to the respective sense electrode.
 19. The implantable device of claim 12, wherein the processor is configured to repeatedly detect whether a locally evoked neural response is present in the neural measurement, in order to obtain a plurality of ratios resulting from repeated application of a given stimulus in order to give a probabilistic indication of the neural response decay to improve the determination of whether a locally evoked response is present.
 20. The implantable device of claim 12, further comprising a signal quality indicator to detect whether any neural activity is present to avoid false positives. 